Corporate logos and the like are often affixed to textiles such as shirts, sweatshirts, caps, shorts, outerwear, pants, etc. Quite often, the corporate logos are screen printed on the garments. However, corporate logos on sewn-on patches or embroidered corporate logos are normally used on more expensive garments.
The invention arose during efforts by the assignee to develop practical means for screen printing raised, three-dimensional images on textile and non-textile substrates as an alternative to sewn-on patches and/or embroidery. In developing a raised image screen printing technique, it was important to develop a technique that can be used reliably on conventional production equipment such as automatic indexing screen printing machines. Such screen printing machines are typically used to print multi-colored images on substrate material such as T-shirts, sweatshirts, posters, etc. In these machines, the substrate is mounted on one of several palettes that typically extend radially from an indexed turret. The turret is then rotated to move the palettes among a plurality of print processing stations located peripherally around the turret.
In one aspect, the invention is a method of printing a raised image on a substrate, such as a garment, using an automatically indexing screen printing machine. In accordance with the invention, a prefabricated stencil is attached to a print side of the print screen used at the printing station for the raised image. The print screen is coated with an emulsion that is hardened except in one or more image areas. The stencil has a prefabricated thickness, for example, 400 microns. One or more image openings pass through the stencil. Each of the openings through the stencil has a shape corresponding to the one or more image areas passing through the hardened emulsion coating on the print screen. The one or more image openings through the stencil define one or more three-dimensional image volumes in the stencil for the form of raised image. The raised image is formed by screen printing pigmented paste through the one or more image areas of the print screen and the associated image volume in the stencil onto the substrate. Additional flat or raised layers can be printed on an initial raised image layer to create complex, colorful three-dimensional images. It is normally preferred that the initial raised image layer be heat cured before subsequent printing.
Using the above-described method, raised images having sharp and distinct resolution can be reliably and effectively printed using automatically indexing screen printing machines. Specifically, raised images can be produced having a top surface that is planar as well as peripheral edges that are substantially perpendicular to the top planar surface. In addition, subsequent raised image levels can be built on top of previous raised levels having the same or different shape to enhance image contrast as desired.
In the prior art, puffing agents have been added to ink in order to produce raised images. In puff printing, the puffing agent normally constitutes 60% to 75% by weight of the ink. When the printed substrate is processed through the dryer, the puffing agent forms a gas to raise the ink. While puff printing can provide raised images, it is difficult to print puffed images having sharp, distinct lines. Puffed images are by nature rounded. Also, it is difficult to control the precise image created by puff printing, especially when used to print rather large areas, or when layering levels of puffed printing. In many applications, such as the printing of three-dimensional corporate logos and the like, the invention as described above is far more desirable than puff printing.
The preferred photosensitive material for the stencil is a pure polymer, and the preferred photosensitive emulsion is a pure polymer emulsion that is compatible with the stencil material. The stencil blank is preferably a prefabricated pure polymer thick film having precision controlled thickness (e.g. 400 microns, 700 microns, etc.) as is sometimes used in the electronics manufacturing industry. The pure polymer stencil blank is attached to the print side of the screen using the photosensitive emulsion, and left to dry for a significant amount of time to ensure secure attachment. The print screen is then coated with the photosensitive emulsion, preferably two coats, and then left to dry. Thereafter, the film positive or image mask is placed preferably adjacent the stencil blank. The masked print screen and stencil blank are exposed to UV light from metal halide lamps to harden the exposed polymer emulsion and the exposed portion of the polymer stencil blank. The screen and stencil are then washed to remove non-hardened emulsion and stencil material preferably using water and a soft brush. Using a print screen and stencil created in the manner described can be used to produce raised images of solidified pigmented paste that exhibit unprecedented definition and resolution.
There are other aspects of the invention which facilitate effective printing of raised three-dimensional images of solidified pigmented paste. For instance, it is desirable that the squeegee or squeegees be relatively stiff, and that the lower edge of the squeegees be beveled to help push a high volume of pigmented paste through the image area in the print screen and the image volume of the stencil onto the substrate. Further, it is desirable that the print station employ two squeegees, each opposing one another. The flood squeegee forces pigmented paste into the image volume created by the stencil as the print head is moving forward during the flood stroke of the print head to fill the image volume. The print squeegee continues to push pigmented paste into the image volume and deposits the pigmented paste on the substrate during the print stroke of the print head. In some applications, it may not be necessary to use two squeegees to effectively implement the invention.
Further, the viscosity of the pigmented paste should be controlled to provide stability to the raised image before heat curing or other processing. To this end, a dry powder thickener can be added to a pigmented polyvinyl chloride resin-based ink to provide a pigmented paste having sufficient viscosity. It is also normally important that the print screen and stencil release cleanly from the three-dimensional form of pigmented paste printed onto the substrate after the print squeegee passes the image area during the print stroke. To this end, it is preferred to add a releasing agent to the pigmented paste mixture.
Other features and advantages of the invention should be apparent to those skilled in the art upon inspecting the following drawings and description thereof.